If the New Testament is to believed and they were real people, then Judas betrayed Jesus for money, thirty pieces of silver. He reportedly regretted what he did and hanged himself in grief.

It might be more complicated than that. In recent years there has been discovered a Gnostic gospel, the Gospel of Judas. According to this gospel, Judas had been Jesus' closest disciple. Jesus told Judas ahead of time that he must betray Jesus to the Romans in order for Jesus to fulfill his role as Lamb of God, that if Judas didn't betray Jesus then Jesus would never be able to Save mankind.

Even if we don't accept this gospel as canon (I'm not aware of any religions that do accept it), it does raise an interesting debate. Did Judas need to betray Jesus in order for Jesus to become savior? Was Judas simply fulfilling a role God needed filled? These are theological questions and might be better suited to /r/AcademicBiblical